1. Field of the Invention
The present invention relates to the field of the correction of images from a sensor causing a periodic noise. It is more particularly designed for the correction of the striping defects created in heat cameras through a sensor such as this, and an object of the invention is method for the "destriping" of images by adaptive processing and a device for the implementation of such a method.
2. Description of the Prior Art
The chain for the display of images taken by a heat camera has the following elements: objective--scanning module--detection system--signal processing device--display device.
The detection system is constituted by a matrix sensor formed by a 2D mosaic of n lines of elementary, photovoltaic or photodetective sensors, cooled to 77.degree. K. to reduce the thermal noise to the minimum. In FIG. 1, the detector or sensor mosaic 1 of the reference camera is constituted by n=11 groups of four elementary cells. The groups of cells of two successive lines are offset for technological reasons, and the whole set of groups is in three columns. This configuration enables the analysis of one pixel per group of cells. An optic system 2, formed by mirrors (M1, M2, M3) and an output lens LS, projects an elementary zone A of the image A on the sensor 1. This image A is formed by the objective O of the camera from the scene observed. This zone Z describes the field of observation of the camera by means of a double scanning operation:
a horizontal scanning, prompted by a pyramidal mirror 3 with twelve facets;
a vertical scanning, done with a pivoting plane mirror 4.
The image, with a format of 24.times.36 pk mm.sup.2, is analyzed in 47 bands of 11 lines and divided into 780 pixels per line for 517 useful lines (giving 780 pixels on 11.times.47 lines for the reference camera and, more generally, Q pixels on nxP lines).
Owing to the inhomogeneous response of electronic components (preamplifiers, capacitors or delay lines) associated with the sensor cells, combined with inhomogeneities having optic causes (periodic variations in the sighting axis) and mechanical causes (driving of the pivoting mirror for the vertical scanning), streaks appear on the display device which take the form of the "striping" effect and have the following specific characteristics:
inhomogeneity along the lines in a dome-shaped curve of luminance;
inhomogeneity along the columns from top to bottom of the image;
offset of the domes from one line to the next one;HERE
inhomogeneity in n-line periods on the image.
These characteristics show that the striping defect is chiefly related to the structure of the matrix-shaped detection system and to the associated scanning module with which the heat cameras are fitted out. The analysis of the image in bands of n lines by means of such a system prompts the appearance, on the final image, of zones wherein, for each pixel, the luminance values resulting from the striping defect are superimposed on the luminance values of the fault-free image. All these zones define a striped space.
Different approaches have been put forward to eliminate these striping defects by the "destriping" of the image. These approaches include, in particular:
Yasuoka's destriping method (cf. T. Suzuki & H. Ogura ed., Proceedings of the 1984 International Symposium on Noise and Clutter Rejection in Radars and Imaging Sensors, c IECE 1984), in which the image is modelized on the basis of the following hypothesis: the mean of the luminance level of a "destriped" image line may be expressed as a linear combination of the means of the luminance levels of a band centered on this line.
destriping by defocusing: the sighting line of the camera is tilted, or the objective is masked, so that a pure striped image is made to appear on a uniform gray background to be subtracted from the image delivered by the camera.
Such approaches are not satisfactory:
in the former example, the striping is only partially attenuated and the method prompts the appearance of a secondary striping, in the form of flashes or blurred features, owing to the erroneous interpretation of the breaks in continuity (corresponding to sudden variations of luminance in the landscape);
in the latter example, the proposed approach requires the periodic updating of the correction to be made, whence a periodic interruption of emission incompatible with the continuous use of the camera.